1. Introduction

“We are living in a period of the world's history when the mingling of thousands of kinds of
organisms from different parts of the world is setting up terrific dislocations in nature”Charles Elton, 1958. The ecology of invasions by animals and plants.

An integral part of sustainable forest management includes measures to protect forests from natural threats such as fire, insects and diseases. Increasingly, an additional, and more severe threat, has been affecting the forest sector worldwide - alien invasive species. Alien invasive species are any species that are non-native to a particular ecosystem and whose introduction and spread causes, or are likely to cause, socio-cultural, economic or environmental harm or harm to human health.

The increasing global movement of people and products, though beneficial to many people, is also facilitating the movement of alien species around world. These species may be unintentionally introduced to new environments in shipments of food, household goods, wood and wood products, new and used tires, animal and plant products, containers, pallets, internal packaging materials, and humans. In the absence of their natural predators, competitors and pathogens, they prosper in their new environments and spread at the expense of native species, affecting entire ecosystems.

Not all invasive species have been inadvertently introduced, however. Particularly challenging to natural resource management are non-native species that have been intentionally introduced into an ecosystem to provide economic, environmental or social benefits. Many species of plants, trees and animals have been introduced outside their native ranges as ornamentals, for gardening, or for the pet industry, and these species have escaped to become serious problems in forests and other ecosystems. This is a considerable concern in the forest sector since many of the tree species used for agroforestry, commercial forestry and desertification control are alien or non-native to the area. It is vital to ensure that such species serve the purposes for which they were introduced and do not escape to cause negative effects on native ecosystems.

While the definitions and impacts of invasive species on the forest sector are still debated and need reviewing in the context of forest management, a number of initiatives, programmes and activities have been initiated. Most programmes focus on damage caused to local forest ecosystems, or to particular species or group of species, by a given pest 1over a period of time. There is an overall lack of information on alien invasive species and the forest sector at the global scale. Information sharing is necessary in the planning and implementation of any strategy for the management of alien invasive species.

This review concentrates, but not exclusively, on insects, diseases and woody species that have the potential to become invasive.

1 Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products.